In recent years, the demand of lithium ion battery is enlarged because of enormous capacity and free from memory effect which was widely applied to electronic devices. However, the explosion of Samsung Note7 battery arouses public attention in of lithium ion battery's safety issue. In addition, the capacity of battery will drop during usage time, one battery can only work for two years normally. In this study, we will find caused the explosion and capacity decrease of battery during usage process. The methodology started with thermal shock test to accelerate the aging time of battery. During the testing, the capacity change of battery after each 200 h was recorded. After the test, each component including the protective circuit, cathode and anode were analyzed respectively. From protective circuit aspects, the corrosion of PCB by hydrofluoric acid will cause malfunction of the circuit leading to overcharging or over-discharging. Since the decomposition of SEI on anode leading to exfoliation of graphite. The generation of fresh SEI will consume lithium ion inside the electrolyte leading to capacity decrease. Moreover, the phenomenon why some batteries will be swollen up during usage has been analyzed from chemical reaction aspect. Finally, this study can be used to offer good suggestions to manufacturers in improving the reliability of lithium ion battery.